This invention relates to metal oxide varistors and, more particularly, to a passivating coating for improving the stability and electrical characteristics of metal oxide varistors.
In general, the current flowing between two spaced points is directly proportional to the potential difference between those points. For most known substances, current conduction therethrough is equal to the applied potential difference divided by a constant, which has been defined by Ohm's law to be its resistance. There are, however, a few substances which exhibit non-linear resistance. Some devices, such as metal oxide varistors, utilize these substances and require resort to the following equation (1) to quantitatively relate current and voltage: ##EQU1## where V is the voltage applied to the device, I is the current flowing through the device, C is a constant and .alpha. is an exponent greater than 1. Inasmuch as the value of .alpha. determines the degree of non-linearity exhibited by the device, it is generally desired that .alpha. be relatively high. .alpha. is calculated according to the following equation (2): ##EQU2## where V.sub.1 and V.sub.2 are the device voltages at given currents I.sub.1 and I.sub.2, respectively.
At very low voltages and very high voltages metal oxide varistors deviate from the characteristics expressed by equation (1) and approach linear resistance characteristics. However, for a very broad useful voltage range the response of metal oxide varistors is as expressed by equation (1).
The values of C and .alpha. can be varied over wide ranges by changing the varistor formulation and the manufacturing process. Another useful varistor characteristic is the varistor voltage which can be defined as the voltage across the device when a given current is flowing through it. It is common to measure varistor voltage at a current of 1 milliampere and subsequent reference to varistor voltage shall be for voltage so measured. Still another useful varistor characteristic is the leakage current. This is the dc steady state current through the device when it is exposed to one-half of its varistor voltage. A high leakage current causes wasted power and, if high enough, can cause joule heating of the device, which leads to a higher current. Thus, high leakage current can lead to thermal runaway. The foregoing is, of course, well known in the prior art.
Metal oxide varistors are usually manufactured as follows. A plurality of additives is mixed with a powdered metal oxide, commonly zinc oxide. Typically, four to 12 additives are employed, yet together they comprise only a small portion of the end product, for example less than five to ten mole percent. In some instances the additives comprise less than one mole percent. The types and amounts of additives employed vary with the properties sought in the varistor. Copious literature describes metal oxide varistors utilizing various additive combinations. For example, see U.S. Pat. No. 3,663,458. A portion of the metal oxide and additive mixture is then pressed into a body of a desired shape and size. The body is then sintered for an appropriate time at a suitable temperature as is well known in the prior art. Sintering causes the necessary reactions among the additives and the metal oxide and fuses the mixture into a coherent pellet. Following sintering metallic contacts are applied and leads are fixed to the contacts. Finally, the device is generally encapsulated.
Problems encountered in varistors manufactured by the prior art method include poor stability and a current leakage that renders the device unsuitable for certain applications. Different varistor formulations have been tested in an effort to control the leakage current, but none has heretofore been fully acceptable. Passivating coatings are sometimes used to enhance the stability and reduce the leakage current of the device. However, the heretofore known coatings are not completely successful and, in addition, the constituents of the coating sometimes have an adverse effect on the electrical characteristics of the device.
It is, therefore, an object of this invention to provide a passivating coating for metal oxide varistors that is fully compatible therewith and improves device stability and reduces leakage current thus providing substantially improved devices.